A hydraulic excavator, an example of hydraulic working machine, typically includes a track structure; a swing structure mounted atop the track structure in a swingable manner; and a multi-joint working device (front device), which is connected to the swing structure in a vertically movable manner and composed of a boom, an arm, and a bucket. The track structure, swing structure, and front device constitute a driven component of a drive system mounted on the excavator.
Such a drive system is often of the hydraulic type and commonly includes a prime mover (engine); at least one hydraulic pump driven by the prime mover; and multiple hydraulic actuators driven by the hydraulic fluid discharged from the hydraulic pump. Examples of hydraulic actuators include a hydraulic boom cylinder, a hydraulic arm cylinder, and a hydraulic bucket cylinder, used to actuate a boom, an arm, and a bucket, respectively. Further examples include a hydraulic travel motor for moving a track structure and a hydraulic swing motor for rotating a swing structure relative to the track structure.
In the field of automobiles, on the other hand, the use of hybrid drive systems has been proposed. In a hybrid drive system, an engine drives a generator, and the resultant electric power is partially used to drive an electric motor. The rest of the power is stored on a battery. Thus, when the engine power is low, the hybrid drive system can assist the engine by using the electric power stored on the battery to drive the generator as an electric motor. This hybrid system always allows the engine to operate efficiently, thereby ensuring energy saving and reducing gas emission.
Thus, the application of such hybrid drive systems to hydraulic working machine such as hydraulic excavators is now being proposed, an example of which is a hybrid hydraulic working machine having an electric swing motor. The electric swing motor is advantageous for saving energy because the kinetic energy at the time of braking the electric motor can be regenerated as electric energy (note that in the case of a hydraulic swing motor, such kinetic energy is discharged as heat).
However, heat is generated from such electrical components as an electric swing motor, an electrical storage device for storing and discharging electric energy, and a power control unit (PCU) which is used to control the operation of the electric swing motor and the charging and discharging of the electrical storage device. For this reason, a hybrid hydraulic working machine is provided with a cooling system for cooling its electrical components. Such a cooling system includes a radiator for cooling a coolant and a pump for circulating the coolant, and the cooling system causes the pump to circulate the coolant cooled by the radiator, thereby cooling electrical components.
Widely used LLCs (long-life coolants) tend to increase in viscosity when temperature is low. Such an increased viscosity of a coolant increases line resistance of a cooling circuit. This increases pump loads, resulting in a decrease in energy efficiency. This issue is common to cooling systems used for hybrid automobiles.
The cooling system of Patent Document 1 below is proposed to overcome this issue. This conventional cooling system for an electric automobile aims to achieve stable cooling irrespective of the season and comprises a cooling circuit including a radiator and a pump; multiple switch valves; and a heating circuit including a heater. The cooling system controls the opening and closing of the switch valves based on ambient temperature. Specifically, when ambient temperature is low, those switch valves that have been used to form the cooling circuit at normal temperature are closed, and the other switch valves that have been left closed at that normal temperature are opened to form the heating circuit. The heater (an external heat source) is used to heat a coolant, and the heated coolant is circulated through the heating circuit. With this configuration, the conventional cooling system prevents a decrease in energy efficiency, which is attributable to the coolant viscosity increased at low temperature.